The present invention relates to a method of preparing porous particles using a fugitive substance, and to gas-filled porous particles and their aqueous dispersions.
Air-filled porous particles are valued for their ability to opacify otherwise transparent coatings. This ability to opacify is of particular value when a coating is being applied to a surface of a substrate to hide markings on and coloration of that surface. It is thought that such opacification is achieved in much the same way pigments such as titanium dioxide (TiO2) opacify polymeric coatings into which they are incorporated. Like small particles of TiO2 dispersed in a coating, the air-filled voids scatter light. The scattering of visible light, observable as opacity, is particularly intense when the scattering center, in this case the air-filled pore, is between about 50 nanometers and 1 micron in diameter. The term xe2x80x9chiding powerxe2x80x9d is used herein to refer to the ability of a given coating to hide the substrate surface to which it is applied. An increase in opacity for a given coating will be observed as an increase in hiding power.
When porous particles are dispersed throughout a dry polymeric coating, the pore or pores in each particle typically contain air. Because the air filled pores behave like TiO2, porous particles can completely, or partially replace pigments like TiO2 in coating compositions for which opacity is a requirement. Replacement of inorganic pigments, such as TiO2, with air has obvious financial and environmental advantages associated with it. As a result, porous particles have been commercially successful as materials that provide hiding power to coatings.
Porous particles are typically produced as aqueous dispersions of particles, the pores of which are filled with water. These aqueous dispersions of particles are incorporated into coating compositions, including a variety of paints. Because the refractive index difference between water and polymer is typically much less than that between air and polymer, or other gases and polymer, the coating composition does not achieve full opacity during application to a substrate and formation of a coating until that coating has dried to the extent that the water in the pores has been replaced with air. It is an unfortunate reality that, although the ultimate xe2x80x9cdry hiding powerxe2x80x9d imparted to a coating by porous particles may be such that it would be fully acceptable to a customer, the lower xe2x80x9cwet hiding powerxe2x80x9d observable during application of the coating may dissuade the customer from proceeding to completion. In this way, for example, a coating composition that replaces all or a portion of TiO2 with cheaper, more environmentally desirable air is often rejected as an option based not upon actual performance of the coating once dry, but rather based upon anxiety experienced by the customer during application.
It is, therefore, desirable to prepare aqueous dispersions of porous particles having pores filled with air, or other gaseous material. In that way, wet hiding power similar to the ultimate dry hiding power of a given coating composition can be achieved.
U.S. Pat. No. 5,225,279 discloses a method of forming porous particles by first dispersing droplets of monomer and other hydrophobic substances in water, followed by polymerization. The polymer formed during the polymerization bears acidic groups such as those of carboxylic acid monomers, and phase separation after polymerization is facilitated by neutralization of those acidic groups with base such as, for example, ammonia. The neutralization converts the carboxylic acid groups, most of which are not ionized, into carboxylate salts that are fully ionized. By making the polymer more ionic, and therefore more hydrophilic, in this way, the polymer is rendered less soluble in the hydrophobic component of the droplet, and phase separation occurs readily. This technique, disclosed in detail in U.S. Pat. No. 5,225,279, has the further advantage that the more ionic, more hydrophilic polymer bearing, for example, ammonium carboxylate groups will tend to move to the droplet/water interface, increasing the propensity to form a continuous shell around a single pore containing the more hydrophobic phase. Similarly, polymers bearing salt moieties may be formed by neutralization of basic groups on those polymers. For example, amine functional polymers may be neutralized with hydrochloric acid or acetic acid to form more hydrophilic ionic polymers. Because dispersion of the droplets prior to polymerization is accomplished exclusively by mechanical means, the particle size distribution of both droplets and porous particles is broad. When a volatile solvent is included in the droplets as a hydrophobic substance, polymerization and neutralization produces porous particles which may be separated from the aqueous phase and dried, creating air-filled particles having a broad particle size distribution.
U.S. Pat. No. 5,976,405 discloses porous particles having particle sizes in the range of 0.150 micron to 15 microns, and very narrow particle size distributions that derive from using a low molecular weight seed polymer, itself having a very narrow particle size distribution. This method does not require neutralization after polymerization, although neutralization may be a preferred option in some cases. When a volatile solvent is included in the droplets as a hydrophobic substance, polymerization and phase separation produces porous particles which may be separated from the aqueous phase and dried, creating air-filled particles having a very narrow particle size distribution. Unfortunately, neither U.S. Pat. No. 5,976,405 nor U.S. Pat. No. 5,225,279 disclose air-filled particles dispersed in water.
Displacement by air of a solvent contained in a pore, while a particle is still dispersed in water, is extremely difficult or impossible. Attempts to remove low boiling solvents (i.e., solvents having a normal boiling point of 30xc2x0 C. to 70xc2x0 C.) from the pores of porous particles having continuous shells is very difficult when those particles are dispersed in water, even at reduced pressure and elevated temperature. At reduced pressure, the amount of gas (e.g., nitrogen or air) available to replace the solvent is very limited and the diffusion of the gas through the aqueous phase slows the replacement process further. As a result, solvent escaping from the pore is not easily replaced with gas. The partial vacuum thus created in the pore may then bring about collapse of the particle, with concomitant change in particle shape, and loss of the desired porosity. If, on the other hand, elevated temperatures are used to encourage evaporation of the solvent, it is often the case that the required temperature for efficient evaporation is at least as high as the softening temperature of the polymeric phase, with the result that the desired porous structure is lost.
We have, surprisingly, found that it is possible to prepare aqueous dispersions of porous particles having a polymeric phase filled with at least one gaseous substance, for example, air. The method of preparation of these porous particles involves the use of a fugitive substance during polymerization. The fugitive substance can then be removed smoothly and continuously at temperatures as low as 1xc2x0 C. above the freezing point of the aqueous phase, allowing even porous particles having a polymeric phase which softens at least 5xc2x0 C. above the freezing point of the aqueous phase to be freed of the fugitive substance without loss of porosity, and without loss of shape.
Polymeric particles having pores filled with gas may be prepared according to the method of the present invention in a wide range of particle sizes (PS) spanning 0.015 microns to at least 250 microns, and particle size distributions (PSDs) including broad, narrow, very narrow, monodisperse, bimodal, and multimodal.
The present invention relates to a method of preparing an aqueous dispersion of a plurality of porous particles, wherein said porous particles comprise at least one polymeric phase and a pore filling phase, comprising the steps of:
a) forming a reaction mixture in a closed pressure vessel, said reaction mixture comprising:
i) at least one monomer;
ii) water;
iii) a dispersing agent; and
iv) at least one fugitive substance;
b) forming a plurality of droplets comprising said monomer and said fugitive substance as a dispersed phase in water;
c) polymerizing said monomer to form said polymeric phase;
d) causing said polymeric phase and said pore filling phase to phase separate from one another, forming said porous particles; and
e) reducing the pressure in said pressure vessel to atmospheric pressure; and
wherein said pore filling phase comprises said fugitive substance.
Another aspect of the method of the present invention comprises the further step of at least partially replacing the fugitive substance with a replacement gas.
Yet another aspect of the method of the present invention comprises the step of forming a plurality of seed particles by at least one aqueous emulsion polymerization of at least one seed monomer; and
wherein said reaction mixture further comprises said seed particles.
A still further aspect of the method of the present invention comprises the steps of:
(1) forming a plurality of pre-seed particles by aqueous emulsion polymerization of at least one pre-seed monomer; and
(2) forming a plurality of seed particles by at least one aqueous polymerization of at least one seed monomer in the presence of said pre-seed particles;
wherein the seed polymer formed by said polymerization of said seed monomer has a number average molecular weight of 500 to 50,000; and
wherein said reaction mixture further comprises said seed particles.
In yet another aspect of the method of the present invention, said polymerizing is carried out at a pressure greater than the critical pressure of said fugitive substance and at a temperature greater than the critical temperature of said fugitive substance.
The present invention also relates to an aqueous dispersion of a plurality of porous particles, said porous particles comprising:
a) at least one polymeric phase; and
b) a pore filling phase;
wherein said pore filling phase comprises a substance selected from the group consisting of a fugitive substance, a replacement gas, and combinations thereof.
The present invention additionally relates to a plurality of porous particles, said porous particles comprising:
a) at least one polymeric phase; and
b) a pore filling phase;
wherein said pore filling phase is selected from the group consisting of a fugitive substance, a replacement gas, and combinations thereof; and
wherein the effective glass transition temperature of the polymeric phase is 1xc2x0 C. to 50xc2x0 C.
Used herein, the following terms have these meanings:
A xe2x80x9cdispersed phasexe2x80x9d is any phase dispersed in the aqueous phase. xe2x80x9cDispersed phasesxe2x80x9d include, for example, droplets, porous particles, seed particles, and pre-seed particles. An xe2x80x9caqueous dispersionxe2x80x9d includes an xe2x80x9caqueous phasexe2x80x9d and at least one xe2x80x9cdispersed phasexe2x80x9d.
The xe2x80x9cdroplet mediumxe2x80x9d is the contents of the plural droplets at the start of and during polymerization to form the plural porous particles.
A xe2x80x9cpluralityxe2x80x9d of droplets or particles, including porous particles, refers to a collection of more than one, and usually a large number, of such droplets or particles. The terms xe2x80x9cplural dropletsxe2x80x9d and xe2x80x9cplural particlesxe2x80x9d refer to such collections of droplets and particles, respectively.
The xe2x80x9creaction mixturexe2x80x9d is the total contents of the pressure vessel that is present during the polymerization.
A xe2x80x9cporous particlexe2x80x9d is a particle having at least one polymeric phase and one or more pores.
A xe2x80x9cporexe2x80x9d is a space within the porous particle, the boundaries or walls of which are defined completely, or partially by the polymeric phase of that particle.
The xe2x80x9cfugitive substancexe2x80x9d of the present invention is any substance having a normal boiling point of less than 30xc2x0 C., preferably less than 10xc2x0 C., more preferably less than xe2x88x9210xc2x0 C., and most preferably less than xe2x88x9250xc2x0 C. The fugitive substance may be soluble in water under conditions of polymerization and removal of the fugitive substance, provided that it is also soluble in the droplet medium. It is preferred that the fugitive substance be more soluble in the droplet medium than in the aqueous phase. The solubility of the fugitive substance in water at the temperature of the polymerization is typically less than 30%, preferably less than 10%, more preferably less than 2%, and most preferably less than 0.5% by weight, based on the total weight of water. The fugitive substance of the present invention is further a substance that is not reactive with the other ingredients of the reaction mixture under the conditions utilized for polymerization and removal of the fugitive substance.
A xe2x80x9cpore filling phasexe2x80x9d is a phase that includes a fugitive substance, a replacement gas, or a combination thereof.
The xe2x80x9cnormal boiling pointxe2x80x9d of a substance is the temperature at which the vapor pressure of that substance equals the atmospheric pressure (i.e., 760 mm) at sea level.
A xe2x80x9cdispersing agentxe2x80x9d is a substance that functions to stabilize droplets and porous particles in aqueous media. Surfactants and suspending agents are types of dispersing agents.
A xe2x80x9cporogenxe2x80x9d is that portion of the droplet medium from which the polymeric phase separates during or after polymerization. The xe2x80x9cporogenxe2x80x9d may include a fugitive substance, monomers, other compounds associated with the polymerization, or combinations thereof
A xe2x80x9creplacement gasxe2x80x9d is any substance that is a non-reactive gas under conditions of storage and use of the aqueous dispersion of the plural porous particles, and under conditions of storage and use of the plural porous particles in dry form. The xe2x80x9creplacement gasxe2x80x9d is added to the aqueous dispersion subsequent to polymerization to replace, or partially replace the fugitive substance.
xe2x80x9cParticle sizexe2x80x9d is the diameter of a particle.
The xe2x80x9caverage particle sizexe2x80x9d determined for a collection of droplets or particles, including porous particles, varies somewhat according to method of determination (e.g., by DCP or BI-90, as described herein below), but is approximately, or identically, xe2x80x9cthe weight average particle sizexe2x80x9d, xe2x80x9cdwxe2x80x9d, also described herein below. xe2x80x9cTgxe2x80x9d is the xe2x80x9cglass transition temperaturexe2x80x9d of a polymeric phase. The glass transition temperature of a polymer is the temperature at which a polymer transitions from a rigid, glassy state at temperatures below Tg to a fluid or rubbery state at temperatures above Tg. The Tg of a polymer is typically measured by differential scanning calorimetry (DSC) using the mid-point in the heat flow versus temperature transition as the Tg value. A typical heating rate for the DSC measurement is 20 Centigrade degrees per minute. The Tg of various homopolymers may be found, for example, in Polymer Handbook, edited by J. Brandrup and E. H. Immergut, Interscience Publishers. The Tg of a polymer is calculated by using the Fox equation (T. G. Fox, Bull. Am. Physics Soc., Volume 1, Issue No. 3, page 123 (1956)).
xe2x80x9cEffective Tgxe2x80x9d. When a substance having some degree of solubility in a polymer is imbibed by that polymer, the softening temperature of the polymer decreases. This plasticization of the polymer can be characterized by measuring the xe2x80x9ceffective Tgxe2x80x9d of the polymer, which typically bears an inverse relationship to the amount of solvent or other substance contained in the polymer. The xe2x80x9ceffective Tgxe2x80x9d of a polymer containing a known amount of a substance dissolved within is measured just as described above for xe2x80x9cTgxe2x80x9d. When the imbibed substance is a fugitive substance, the DSC instrument must be outfitted with a pressure chamber. Alternatively, an estimate of the xe2x80x9ceffective Tgxe2x80x9d, which may be particularly useful when the effect of a fugitive substance is to be determined, can be made by placing pieces of polymer having a specific non-spherical shape (e.g., rectangular parallelepipeds having length, width, and height of 6 mm, 2 mm, and 3 mm, respectively) in a pressure vessel and contacting the polymer with a known amount of fugitive substance, at an elevated pressure. The contents of the closed pressure vessel can then be heated to a temperature estimated to be near the effective Tg, held at that temperature for a period of time sufficient to allow penetration of the polymer by and equilibration with the fugitive substance (usually approximately one hour), and then cooled. Opening the vessel then reveals whether or not the pieces have curled or perhaps even formed into a spherical geometry. If the pieces have retained their shape, the temperature chosen was below the effective Tg of the plasticized polymer. Pieces that have completely lost their shape indicate that the peak temperature was above the effective Tg of the plasticized polymer. A few repetitions of this process provide a practical estimate of effective Tg.
Molecular Weight. Synthetic polymers are almost always a mixture of many different molecular weights, i.e. there is a xe2x80x9cmolecular weight distributionxe2x80x9d, abbreviated xe2x80x9cMWDxe2x80x9d. For a homopolymer, members of the distribution differ in the number of monomer units which they contain. This idea also extends to copolymers. Given that there is a distribution of molecular weights, the most complete characterization of the molecular weight of a given sample is the determination of the entire molecular weight distribution. This characterization is obtained by separating the members of the distribution and then quantitating the amount of each that is present. Once this distribution is at hand, there are several summary statistics, or moments, which can be generated from it to characterize the molecular weight of the polymer.
The two most common moments of the distribution are the xe2x80x9cweight average molecular weightxe2x80x9d, xe2x80x9cMwxe2x80x9d, and the xe2x80x9cnumber average molecular weightxe2x80x9d, xe2x80x9cMnxe2x80x9d. These are defined as follows:
Mw=xcexa3(WiMi)/xcexa3Wi=xcexa3(NiMi2)/xcexa3NiMi
Mn=xcexa3Wi/xcexa3(Wi/Mi)=xcexa3(NiMi)/xcexa3Ni
where:
Mi=molar mass of ith component of distribution
Wi=weight of ith component of distribution
Ni=number of chains of ith component
and the summations are over all the components in the distribution. Mw and Mn are typically computed from the MWD as measured by Gel Permeation Chromatography (see the Experimental Section).
It may be advantageous to carry out the polymerization in the plural droplets, and in some cases even remove much of the fugitive substance from the resultant porous particles, under conditions above the critical point of the fugitive substance. Under these conditions, the fugitive substance behaves as a supercritical fluid. Used herein, the terms xe2x80x9csupercriticalxe2x80x9d and xe2x80x9csupercritical fluidxe2x80x9d have their conventional meanings in the art. A supercritical fluid (xe2x80x9cSCFxe2x80x9d) is a substance above its critical temperature and critical pressure (i.e., its xe2x80x9ccritical pointxe2x80x9d). Compressing a gas normally causes a phase separation and the appearance of a separate liquid phase. However, if the fluid is in a supercritical state, compression will only result in density increases. Above the critical point, further compression does not cause formation of a liquid. The critical temperatures, critical pressures, and therefore critical points of a wide range of substances have be determined or calculated, and are available in such references as Lange""s Handbook of Chemistry, 14th Edition, John A. Dean editor, McGraw-Hill, pp. 134 to 147 and Phase Behavior of Polymers in Supercritical Fluid Solvents, Chem. Rev., 1999, 99, pp. 565-602.
As is appreciated in the art, all gases have a xe2x80x9ccritical temperaturexe2x80x9d above which they cannot be liquified by increasing pressure. Further, there is a xe2x80x9ccritical pressurexe2x80x9d defined as the minimum pressure required to liquify a gas at its critical temperature. For example, carbon dioxide can exist in a solid state (commonly called dry ice), a liquid state, or a gaseous state. However, carbon dioxide may also exist in a supercritical state, a form of matter in which its liquid and gaseous states are indistinguishable from one another. For carbon dioxide, the critical temperature is 31xc2x0 C. and the critical pressure is 1070 psi (=7,376 kilopascals, kPa). Therefore, carbon dioxide exists as a supercritical fluid at temperatures above 31xc2x0 C. if the pressure is at least 7,376 kPa. Similarly, any substance will exist as a supercritical fluid at temperatures above its critical temperature if the pressure is at least equal to its critical pressure, unless of course that substance decomposes at a temperature below what would have been the critical temperature had the substance been more stable.
In the present invention, the droplet medium includes a fugitive substance, so it is necessary that the reaction be carried out in a closed pressure vessel. Pressure vessels are well know to those skilled in the art of performing chemical reactions at elevated pressure. Typically the pressure vessel is provided with: ability to withstand reaction conditions without leaking or rupturing; means to agitate its contents; means to heat and cool its contents; means to pressurize and depressurize; means to add ingredients (i.e., addition ports, delivery lines, and reservoirs for ingredients to be added, all designed to withstand the pressures of operation without rupture or leakage); and, if addition of ingredients is required when the pressure inside the vessel is above one atmosphere, means to deliver those ingredients against that interior pressure. The heating means may be, for example, an electric heating furnace to heat the reaction mixture to the desired temperature, and the mixing means includes stirrers such as paddle stirrers, impeller stirrers, or multistage impulse countercurrent agitators, blades, and the like. Delivery of ingredients to a vessel, the inside of which is at elevated pressure, is typically achieved by pressurizing the delivery line to a pressure greater than that of the vessel""s interior. This pressurization is typically accomplished by inserting a pressure pump into the delivery line between the ingredient reservoir and the pressure vessel, or by pressurizing the reservoir and delivery line with a compressed inert gas such as, for example, nitrogen or argon.
The polymerization reaction may be carried out at a temperature of 50xc2x0 C. to 200xc2x0 C., and is typically carried out at a temperature of xe2x88x9220xc2x0 C. to 150xc2x0 C. Suitable antifreeze agents, such as ethylene glycol may be added to the aqueous phase of the reaction mixture to avoid freezing the aqueous phase during reactions which are conducted at temperatures below the freezing point of the aqueous phase in absence of the antifreeze agents. The reaction may be carried out at a pressure ranging from about 100 kPa to about 300,000 kPa, and is typically carried out at a pressure of between about 3,000 kPa and about 70,000 kPa. The polymerization can be carried out batchwise or continuously with thorough mixing of the reactants in any appropriately designed high pressure reaction vessel, or tubular reaction vessel. Components of the reaction mixture other than the fugitive substance may be added to the pressure vessel before or after the vessel is closed. The polymerization may be carried out, for example, by adding monomer, seed polymer, suspending agent, initiator if necessary, and water to the pressure vessel. The vessel would then be closed, pressurized with a fugitive substance, and the contents brought to the polymerization temperature and pressure. Alternatively, only a part of the reaction mixture may be introduced into a pressure vessel and heated to the polymerization temperature and pressure, with additional reaction mixture being pumped in at a rate corresponding to the rate of polymerization. In another possible procedure, a portion of the monomers are initially taken into the closed pressure vessel in the total amount of fugitive substance and the remainder of the monomers or co-monomers are pumped into the pressure vessel together with the initiator at the rate at which the polymerization proceeds. When the polymerization is complete, the aqueous dispersion of porous particles may be brought to atmospheric pressure by, for example, venting the reactor. The porous particles may, for example, be further utilized: as an aqueous dispersion; as an aqueous dispersion combined with other aqueous dispersions containing, for example, a dispersed polymer different from the porous polymer of the present invention; as an isolated powder; and as part of a composite (e.g., a coating) with other materials. Any suitable means of separating the polymer from the fugitive substance and aqueous phase may be employed. Typically, according to the process of the present invention, the fugitive substance is vented to a recycle system and replaced by air. In the event that the fugitive substance is carbon dioxide, the carbon dioxide may be collected for recycle or vented to the atmosphere.
In the method of the present invention, the ingredients that, taken together, form the reaction mixture are added to the pressure vessel. These ingredients include monomer, water, initiator if necessary, dispersing agent, and fugitive substance. Other optional ingredients include a polymeric seed and a transport agent.
Fugitive substances useful in the present invention include, for example, 2,2-dimethylypropane (9.5xc2x0 C.), dichlorofluoromethane (8.9xc2x0 C.), cis-2-butene (3.73xc2x0 C.), trans-2-butene (0.3xc2x0 C.), 1,2-dichlorotetrafluoroethane (3.8xc2x0 C.), butane (xe2x88x920.5xc2x0 C.), 1-butene (xe2x88x926.5xc2x0 C.), 1,1,2,2-tetrafluoroethane (xe2x88x9223xc2x0 C.), dimethyl ether (xe2x88x9224.8xc2x0 C.), 1,1-difluoroethane (xe2x88x9225xc2x0 C.), 1,1,1,2-tetrafluoroethylene (xe2x88x9226.4xc2x0 C.), hexafluoropropylene (xe2x88x9228xc2x0 C.), octafluoropropane (xe2x88x9236xc2x0 C.), chlorodifluoromethane (xe2x88x9240.7xc2x0 C.), propane (xe2x88x9242.1xc2x0 C.), propylene (xe2x88x9248xc2x0 C.), pentafluoroethane (xe2x88x9248.6xc2x0 C.), difluoromethane (xe2x88x9251.6xc2x0 C.), sulfur hexafluoride (xe2x88x9263.8xc2x0 C., sublimes), hexafluoroethane (xe2x88x9278xc2x0 C.), carbon dioxide (xe2x88x9278xc2x0 C., sublimes), chlorotrifluoromethane (xe2x88x9281.5xc2x0 C.), trifluoromethane (xe2x88x9284xc2x0 C.), ethane (xe2x88x9288xc2x0 C.), ethylene (xe2x88x92104xc2x0 C.), tetrafluoromethane (xe2x88x92130xc2x0 C.), and methane (xe2x88x92161.4xc2x0 C.). The preferred fugitive substance is a substance selected from the group consisting of 2,2-dimethylypropane, dichlorofluoromethane, 1,2-dichlorotetrafluoroethane, butane, 1,1,2,2-tetrafluoroethane, dimethyl ether, 1,1-difluoroethane, octafluoropropane, chlorodifluoromethane, propane, pentafluoroethane, difluoromethane, sulfur hexafluoride, hexafluoroethane, carbon dioxide, chlorotrifluoromethane, trifluoromethane, ethane, tetrafluoromethane, methane, and combinations thereof. The more preferred fugitive substance is a substance selected from the group consisting of difluoromethane, hexafluoroethane, carbon dioxide, chlorotrifluoromethane, trifluoromethane, ethane, tetrafluoromethane, methane, and combinations thereof. The most preferred fugitive substance is carbon dioxide. Solutions of monomer in the fugitive substance are formed as a plurality of droplets dispersed in water. When the polymer is formed by free radical polymerization of ethylenically unsaturated monomers, a free radical initiator will be one of the ingredients. The initiator may be soluble or partially soluble in the water. However, the initiator is, preferably, chosen to be more soluble in the droplets than in the aqueous phase. The initiator is further chosen to decompose sufficiently quickly at the polymerization temperature that the polymerization is complete within several hours. It is a requirement of the present invention that the polymer be substantially insoluble in the fugitive solvent at some temperature above the freezing point of the aqueous phase so that, at some point during, or subsequent to the polymerization, it will form a separate polymeric phase. In this way, porous particles are formed.
Polymerization of monomers occurs primarily within the droplet medium, resulting in formation of a polymer. The polymer must then be substantially insoluble in the fugitive substance so that it can phase separate from the fugitive substance to create the structure of the porous particle. If the polymerization temperature is above the Tg of the polymer being produced, it is then necessary to reduce the temperature to below the effective Tg of the polymeric phase so that the polymeric structure will become rigid and then remain intact during and after removal of the fugitive substance by, for example, release of pressure. If the polymerization temperature is below the effective Tg of the polymer being produced, the fugitive substance may be removed at the polymerization temperature, or at any temperature below the effective Tg of the polymer.
Whether the polymerization is carried out above the critical point of the fugitive substance or not, much of the fugitive substance may be removed from the pressure vessel under supercritical conditions, provided the temperature of the contents of the pressure vessel is maintained, or adjusted to a temperature above the critical temperature of the fugitive substance. At some point, of course, enough fugitive substance will have been vented that the pressure in the pressure vessel may drop below the critical pressure, and supercritical conditions will no longer obtain. It is not a requirement of the present invention that removal of the fugitive substance, either partial or complete, be carried out predominantly under supercritical conditions. In fact, it is not a requirement that removal of the fugitive substance be carried out under supercritical conditions at all. It is, however, preferred to carry out the polymerization above the critical point of the fugitive substance. It is further preferred to carry out the removal of the fugitive substance at least partially under supercritical conditions.
Phase separation within the droplets, during or after polymerization, must occur in the method of the present invention to produce porous particles. A discussion of the forces causing phase separation in systems involving polymers, and the structures that result, may be found in an article by Tsai and Torkelson, Macromolecules 1990, vol. 23, pp. 4983-4989. Two of the categories for the process of phase separation in systems involving polymers are defined as follows in the art of phase separation, and either one or both are believed to be operative in the method of the present invention. The first category refers to formation of the polymeric phase by phase separation of the polymer from the droplet medium during polymerization. This phenomenon is known as xe2x80x9cpolymerization induced phase separationxe2x80x9d, the acronym for which is xe2x80x9cPIPSxe2x80x9d. The second category refers to a polymer that is soluble in the droplet medium at one temperature, but insoluble at another, e,g., lower temperature, such that cooling the droplet medium after polymerization causes formation of a separate polymeric phase. This phenomenon is known as xe2x80x9cthermally induced phase separationxe2x80x9d (xe2x80x9cTIPSxe2x80x9d). Used herein, a TIPS process would occur if the polymer were soluble, or partially soluble in the fugitive substance at the temperature of polymerization, yet insoluble at a lower temperature. In that case, the TIPS process could be caused to occur by cooling the contents of the pressure vessel to a temperature at which the polymer would phase separate, and removing the fugitive substance. Without wishing to be bound by theory, it is believed that, provided conditions during or after polymerization result in separation of the polymer from the droplet medium according to PIPS, TIPS, or some combination thereof, porous particles will be formed.
Formation of the polymer may be accomplished by free radical polymerization of ethylenically unsaturated monomers or by condensation polymerization of reactive monomer pairs. Formation of the polymer may also occur by metallocene, atom transfer, or by any other suitable method of polymerization amenable to the conditions just described.
Estimation of whether a given polymer will be soluble in a given fugitive substance may be made according to the well-known methods delineated in D. W. Van Krevelen, Properties of Polymiers, 3rd Edition, Elsevier, pp. 189-225, 1990. For example, Van Krevelen defines the total solubility parameter (xcex4t) for a substance by:
xcex4t2=xcex4d2+xcex4p2+xcex4h2,
where xcex4d, xcex4p, and xcex4h are the dispersive, polar, and hydrogen bonding components of the solubility parameter, respectively. Values for xcex4d, xcex4p, and xcex4h have been determined for many solvents and polymers, and can be estimated using the group contribution methods of Van Krevelen. For example, to estimate whether a given polymer will be soluble in a given fugitive substance, one calculates xcex4t2 for the polymer and xcex4t2 for the fugitive substance. If the difference between the two, xcex94xcex4t2, is greater than 25 (i.e., xcex94xcex4t greater than 5), then the polymer should be insoluble in the fugitive substance. These calculations may be used to estimate whether the polymer being formed during polymerization will be insoluble in the fugitive substance and, if so, whether it will phase separate from the droplet medium while unreacted monomer is still present in that droplet medium, or whether that monomer must first be substantially consumed before phase separation will occur.
Separation of the polymeric phase from the other components of the droplet either during or after polymerization produces a porous particle that includes a polymeric phase and one or more pores. A common morphology for the porous particle is one in which a continuous polymeric shell surrounds a single pore. However several other possible morphologies may occur dependent upon the interplay of thermodynamic and kinetic forces within the droplet as it becomes a particle during and after polymerization. The resultant porous particle may have a discontinuous polymeric shell (i.e., a shell having one or more holes) surrounding a single pore. Alternatively, the porous particle may be a continuous or discontinuous polymeric shell around multiple pores that themselves may be closed or may open into one another. The porous particle may, instead, contain the polymeric phase and one or more pores intertwined in a lacy bicontinuous structure. Still another morphology is one in which the polymeric phase separates into one hemisphere of the particle and the fugitive substance separates into the other hemisphere. It is further possible to prepare porous particles by more than one polymerization step. For example, a single pore could be surrounded by multiple concentric polymeric shells, each prepared in a separate polymerization step.
When the polymer formed during the polymerization bears acidic groups such as carboxylic acid, its solubility in the droplet medium may be decreased, thereby augmenting phase separation, by neutralizing the acidic moieties with base such as, for example, ammonia, creating anionic moieties. In this example, the ammonium carboxylate groups that are formed increase the hydrophilicity of the polymer, making it less soluble in the droplet medium. This technique, disclosed in detail in U.S. Pat. No. 5,225,279, has the added advantage that the more hydrophilic polymer bearing the ammonium carboxylate groups will tend to move to the droplet/water interface, increasing the propensity to form a continuous shell around a single pore. Similarly, polymers bearing cationic moieties may be formed by neutralization of basic groups if such groups are present on a polymer chain. For example, amine functional polymers may be neutralized with hydrochloric acid or acetic acid to form more hydrophilic ammonium functional polymers.
The method of the present invention can produce porous particles having sizes in the range 0.150 micron to 250 microns. Herein, xe2x80x9cmicronxe2x80x9d and the symbol xe2x80x9cxcexcxe2x80x9d are used interchangeably. When particles of size 0.150xcexc to 1.0xcexc are formed, techniques known to those skilled in the art of aqueous emulsion polymerization are used to stabilize droplets, to stabilize particles, and to establish average particle size and particle size distribution. In particular, emulsion droplets and particles are stabilized by dispersing agents known as surfactants. When particles of size greater than 1.0xcexc to 250xcexc are formed, techniques known to those skilled in the art of aqueous suspension polymerization are used to stabilize droplets, to stabilize particles, and to establish average particle size and particle size distribution. In particular, droplets and particles of size greater than 1.0xcexc to 250xcexc are stabilized by dispersing agents known as suspending agents. Of course, surfactants may have some utility for droplet and particle stabilization, though usually diminished, above 1.0xcexc, and suspending agents may have some utility, though usually diminished, at and below 1.0xcexc.
Surfactants useful in the method of the present invention are well known to those skilled in the art, and may be found in, for example, Porter, M. R., Handbook of Surfactants, Chapman and Hall, New York, 1991. Examples of useful surfactants for the present invention include ionic surfactants such as, for example, sodium lauryl sulfate, dioctylsulfosuccinate, sodium polyoxyethylene lauryl ether sulfate, sodium dodecyl benzenesulfonate; and non-ionic surfactants such as, for example, glycerol aliphatic esters, polyoxyethylene aliphatic esters, polyoxyethylene alcohol ethers; and stearic acid monoglyceride. Amphoteric surfactants may also be useful. Amphoteric surfactants bear both acidic and basic functionality and are well known in the art (see, for example, Amphoteric Surfactants, ed. B. R. Bluestein and C. L. Hilton, Surfactant Series Vol. 12 Marcel Dekker New York, N.Y.(1982)). Fluorinated surfactants such as perfluoro-octanoic acid and salts thereof are also useful, particularly for stabilization of fluoropolymers. Silicon surfactants are also useful, especially for stabilizing siloxane polymers. In addition, monomeric surfactants may be incorporated into the polymer chain during polymerization. For example, these monomeric surfactants impart covalently bound surfactant functionality to polymers: nonylphenoxypropenylpolyethoxylated sulphate, sodium alkyl allyl sulfosuccinate, allyl ammonium lauryl 12 EO phosphate, allyl ammonium linear dodecyl benzene sulfonate, and allyl ammonium lauryl sulfate.
Suspending agents are also well known in the art. Suspending agents are typically water soluble polymers including, for example, polyvinyl alcohol, poly(N-vinylpyrrolidone), carboxymethylcellulose, gelatin, hydroxyethylcellulose, r partially saponified polyvinyl acetate, polyacrylamide, polyethylene oxide, polyethyleneimine, polyvinylalkyl ethers, polyacrylic acid copolymers of polyacrylic acid, and polyethylene glycol.
Choice of size for the porous particle is primarily a function of desired enduse properties. For example, when particles are desired for their ability to scatter visible light, it may be desirable that their pores have sizes in the range of 0.050xcexc to 1.0xcexc. In cases wherein each particle contains a single pore having its size in that range, the preferred size of each particle would be in the range 0.150 to 2.0xcexc.
In the method of the present invention, an aqueous dispersion of porous particles may be formed wherein the pores contain a pore filling phase which may include the fugitive substance, a replacement gas, or combinations thereof. Under ambient conditions, the contents of the pores may remain gaseous for an extended period of time. The average particle size of the porous particles may be 0.15xcexc to 250xcexc. It is preferred that the average particle size of the porous particles is 0.15xcexc to 15xcexc, more preferably, 0.15xcexc to 10xcexc, and most preferably, 0.15xcexc to 5xcexc. The porous particles may further be separated from the aqueous dispersion, and dried.
Just as a wide range of particle sizes may be accessed by the method of the present invention, the method may be used to produce porous particles having a wide variety of particle size distributions. Herein, the term xe2x80x9cparticle size distributionxe2x80x9d and the acronym xe2x80x9cPSDxe2x80x9d are used interchangeably. Polydispersity is used in the art as a measure of the breadth of the PSD. More generally, xe2x80x9cpolydispersityxe2x80x9d is a construct of applied mathematics that may be used to describe the distribution of sizes of any measurable feature common to a plurality of items. Examples of distributions that may be described in this way include the lengths of polymer chains (i.e., molecular weights) and the diameters of particles (i.e., particle sizes). Used herein, xe2x80x9cpolydispersityxe2x80x9d is a description of the distribution of particle sizes for the plural particles of the invention. As such, xe2x80x9cpolydispersityxe2x80x9d and xe2x80x9cPSD polydispersityxe2x80x9d are used interchangeably. PSD polydispersity is calculated from the weight average particle size, dw, and the number average particle size, dn, according to the formulae:
PSD Polydispersity=(dw)/(dn),
where dn=xcexa3nidi/xcexa3ni 
dw=xcexa3nididi/xcexa3nidi, and
where ni is the number of particles having the particle size di 
The term xe2x80x9cmonodispersexe2x80x9d refers to a particle size distribution having a polydispersity of exactly 1. If, for example, each of 1000 particles had a particle size of exactly 0.454xcexc, the PSD polydispersity would be 1.000. The average particle size of particles of aqueous dispersions may be determined by light scattering techniques such as are employed by the Brookhaven BI-90 Particle Sizer; by sedimentation methods such as are employed by the Chemical Process Specialists Disc Centrifuge Photosedimentation (DCP) unit (see the Examples section); by optical microscopy and by scanning electron microscopy (SEM). The selection of method is dependent upon the particle size being measured. PSD polydispersity may be conveniently measured for particles having sizes in the range of 0.01 micron to 40 microns using the DCP unit (see the Experimental Section).
Used herein, xe2x80x9cbroad PSDxe2x80x9d, xe2x80x9cnarrow PSDxe2x80x9d, and xe2x80x9cvery narrow PSDxe2x80x9d are defined as having PSD polydispersities of greater than 1.3, greater than 1.1 to 1.3, and 1.000 to 1.1, respectively. The methods of producing polymeric particles having broad, narrow, and very narrow PSDs are well known in the art of aqueous emulsion and aqueous suspension polymerization. In addition, bimodal and multimodal particle size distributions may be formed by well known methods, including, for example, the use of two or more types of seed particle, each having a different average particle size. Of course, aqueous dispersions and isolated porous particles having bimodal and multimodal PSDs may also be prepared after polymerization by combining separately prepared aqueous dispersions of porous particles, isolated porous particles, or combinations thereof. Combinations of particles differing in composition may also be prepared in this way.
Methods of polymerization that create porous polymeric particles having broad particle size distributions (i.e., PSD polydispersities of 1.3 to at least 10) can be found in U.S. Pat. No. 5,225,279, and are well known in the art. The methods employ mechanical agitation to break up large droplets into smaller droplets. Mechanical agitation inherently produces a broad distribution of particle sizes because there exists no single-sized locus of droplet and particle formation (i.e., no polymeric seed) within the reaction mixture.
When narrow PSDs are desired, it is necessary to first prepare polymeric seeds by methods well known in the art. Those polymeric seed particles are smaller than the porous particles that are seeded by them. The seed particles are themselves formed by aqueous emulsion polymerization from seed monomers in the presence of relatively large amounts of surfactant, with the result that they have narrow PSDs. Used herein, a monomer polymerized to from a seed particle is called a xe2x80x9cseed monomerxe2x80x9d. Although a variety of techniques, such as condensation polymerization, may be used to prepare polymeric seed particles, the preferred method of polymerization is by free radical initiation of monoethylenically unsaturated monomers. Multi-ethylenically unsaturated monomers may also be included at levels of 0.01% to 5%, preferably 0.01 to 0.5% by weight, based on total weight of monomers polymerized to make the seed particles. Used herein, the term xe2x80x9cethylenically unsaturated monomerxe2x80x9d may include both xe2x80x9cmonoethylenically unsaturated monomerxe2x80x9d and xe2x80x9cmulti-ethylenically unsaturated monomerxe2x80x9d. Alternatively, monomers suitable for condensation polymerization may also be used. Examples of suitable monoethylenically unsaturated, multi-ethylenically unsaturated, and condensation monomers can be found herein below. Monomers added to aqueous systems in which the polymeric seeds are dispersed are imbibed into the seeds, swelling them. For thermodynamic reasons, each polymeric seed particle imbibes an amount of monomer, fugitive substance, and other ingredients, such as initiator, proportional to the amount of seed polymer contained in it. In this way, the PSD of the porous particles formed when the monomers contained in the droplets are polymerized is similar to the PSD of the polymeric seed particles used to nucleate droplet formation. As a consequence, seeding a droplet medium with polymeric seed particles having a narrow PSD results in preparation of larger polymer particles having similarly or identically narrow PSDs. Methods using seed prepared in this way are capable of producing plural porous particles having average particle sizes of 0.150xcexc to 250xcexc, preferably 0.150xcexc to 15xcexc, more preferably 0.150xcexc to 5xcexc, and most preferably 0.150xcexc to 1.0xcexc. The porous particles thus formed may have PSD polydispersities of 1.05 to 1.3, more preferably 1.05 to 1.2, most preferably 1.1 to 1.2.
When very narrow PSDs are desired, it is necessary to first prepare polymeric xe2x80x9cpre-seed particlesxe2x80x9d from which polymeric xe2x80x9cseed particlesxe2x80x9d can subsequently be prepared. The methods of preparation of both pre-seed and seed particles are described in U.S. Pat. No. 5,237,004 and U.S. Pat. No. 5,976,405, and are well known in the art of emulsion polymerization. Prior to polymerization, droplets containing pre-seed monomer are preferably formed in the presence of relatively high levels of surfactant to assure that the droplets have a very narrow PSD. Polymerization of the pre-seed monomer in these droplets results in polymeric pre-seed particles having a very narrow PSD. Used herein, a monomer polymerized to form a pre-seed particles is called a xe2x80x9cpre-seed monomerxe2x80x9d. Although a variety of techniques, such as condensation polymerization, may be used to prepare polymeric pre-seed particles, the preferred method of polymerization is by free radical initiation of ethylenically unsaturated monomers. When this polymerization is carried out by free radical initiation, at least one ethylenically unsaturated monomer is, preferably, used. The PSD polydispersity of the plural pre-seed particles is 1.000 to 1.2, preferably 1.000 to 1.1, more preferably 1.000 to 1.05, and most preferably 1.000 to 1.01. The pre-seed particles have an average particle size of, preferably, 0.020xcexc to 0.200xcexc, more preferably, 0.030xcexc to 0.100xcexc, and, most preferably, 0.40xcexc to 0.70xcexc. The pre-seed particles are then used as the locus of seed monomer droplet formation in the preparation of low molecular weight seeds having very narrow PSDs. Although a variety of techniques, such as condensation polymerization, may be used to prepare these polymeric seed particles, the preferred method of polymerization is by free radical initiation of ethylenically unsaturated monomers. When this polymerization to form the seed particles is carried out by free radical initiation, at least one ethylenically unsaturated monomer is, preferably, used. A chain transfer agent is present in those droplets during polymerization at a level that assures the number average molecular weight (Mn) of the polymer produced in the polymerization to form the seed particles is 500 to 50,000, preferably 1,000 to 20,000, more preferably 1,000 to 10,000, and most preferably 1,000 to 5,000. All ranges cited herein are inclusive and combinable. The PSD polydispersity of the plural seed particles formed in polymerization seeded by the pre-seed particles is 1.000 to 1.2, preferably 1.000 to 1.1, more preferably 1.000 to 1.01, and most preferably 1.000 to 1.005.
Chain transfer agents such as, for example, mercaptans, polymercaptans, and polyhalogen compounds may optionally be added to the monomers in order to moderate molecular weight. Specific examples include alkyl mercaptans such as t-dodecyl mercaptans and hexanethiol; alcohols such as isopropanol, isobutanol, lauryl alcohol, and t-octyl alcohol; and halogenated compounds such as carbon tetrachloride, tetrachloroethylene, and trichlorbromoethane. For forming the seed particles, the amount of chain transfer agent required may be from about 5 percent to about 20 percent by weight based on the total weight of monomer being polymerized, although amounts above 20 percent may be required depending on the molecular weight desired. Chain transfer agents, typically at levels of 6 percent or less by weight based on the total weight of monomer being polymerized, may also be used to regulate molecular weight during polymerization to produce the polymer that will form the polymeric phase of the porous particle.
The low molecular weight seed particles thus produced typically have average particle sizes of 0.50xcexc to 0.800xcexc, and are preferably capable of imbibing up to 1000 times their own weight in monomers and other ingredients to produce droplets that are then polymerized to form polymeric particles having similar, and very narrow PSDs. It is convenient to use low molecular weight seed particles of size 0.100xcexc to 0.800xcexc to prepare polymeric particles, including the porous particles of the present invention, of size greater than 0.150xcexc to 3.0xcexc. Preparation of porous particles of average PS greater than 3.0xcexc may first require preparation of a larger seed. Larger low molecular weight seed particles may be prepared from smaller low molecular weight seed particles by carrying out one or more additional polymerizations of seed monomer in the presence of sufficient suspending agent to stabilize the larger seed particles being formed, and in the presence of sufficient levels of chain transfer agent that polymer of the desired molecular weight is formed. Low molecular weight seed particles of average PS preferably greater than 0.500xcexc to 10xcexc, more preferably greater than 0.500xcexc to 5xcexc, and most preferably greater than 0.500xcexc to 3xcexc can be made in this manner. The preparation of seed particles having an average PS of 1.0xcexc or more requires that a suspending agent, such as those already describe supra, be added, typically at 0.1 to 5 weight percent, based on total weight of seed particles being prepared. Use of a suspending agent is necessary to stabilize aqueous dispersions of seed particles having diameters of 1.0xcexc or more.
When seed particles are used in the method of the present invention, the monomer, fugitive substance, and other ingredients that will be components of the droplet medium during polymerization are added to an aqueous reaction mixture along with the seed particles having a particular PSD, either narrow or very narrow. The monomer, fugitive substance, and other substances may be added to the pressure vessel individually or in combination, and it is often advantageous to emulsify them with water and surfactant prior to, or after addition. These ingredients then move through the aqueous phase and become imbibed into the seed particles, swelling them. For thermodynamic reasons, each polymeric seed particle imbibes an amount of monomer, fugitive substance, and other substances proportional to the amount of seed polymer contained in it. In this way, the PSD of the porous particles formed when the monomers contained in the droplets are polymerized is similar to the PSD of the seed particles used to nucleate droplet formation. As a consequence, seeding a droplet medium with polymeric seed particles having a very narrow PSD results in preparation of larger porous particles having similar, or identical, very narrow PSDs.
It is very difficult or impossible for a substance that is insoluble in water to move through the aqueous phase so that it can reach the surface of the seed particle and be imbibed into that seed particle. Examples of substances that are insoluble in water include most fluorinated monomers, many silicon containing monomers, and some fugitive substances. Used herein, a substance that is water insoluble has a water solubility at 25xc2x0 C. of less than 1 percent by weight, based on the weight of water. In such cases, it may be advantageous to add a transport agent to the pressure vessel. The use of transport agents is described in detail in U.S. Pat. No. 5,976,405. Used herein, a xe2x80x9ctransport agentxe2x80x9d is a substance that is soluble in water, yet has the ability to complex weakly or strongly with the insoluble substance. The complex that forms is sufficiently soluble in water so that the insoluble substance is transported across the aqueous phase and released at the seed particle. Common solvents and monomers may be useful as transport agents. Particularly useful transport agents are macromolecular organic compounds having a hydrophobic cavity. A xe2x80x9cmacromolecular organic compound having a hydrophobic cavityxe2x80x9d is a polymeric molecule, typically cylindrical or approximately cylindrical, which typically has a hydrophilic exterior but has a hydrophobic interior. Such a compound may be used to transport hydrophobic substances (e.g., fluorinated monomers, silicon containing monomers, and fugitive substances that are insoluble in water) through an aqueous environment, and it may even be used to transport substances that have solubilities in water of 0.001 percent or less. Macromolecular organic compounds having a hydrophobic cavity, useful in the method of the present invention, include cyclodextrin and derivatives thereof; cyclic oligosaccharides having a hydrophobic cavity, such as cycloinulohexose, cycloinuloheptose, and cycloinuloctose; calyxarenes; and cavitands.
If a transport agent is used and the transport agent is macromolecular, cyclodextrins (i.e., cyclodextrin and its derivatives) are the preferred macromolecular organic compounds. The selection of cyclodextrin and derivatives thereof useful in the method of the present invention is determined by the solubility of the cyclodextrin and cyclodextrin derivatives in the aqueous phase, by the similarity in size between the insoluble substance and the hydrophobic cavity of the cyclodextrin, and by the solubility of the species formed by the association of the transport agent with the insoluble substance. Suitable cyclodextrins useful in the method of the present invention include: xcex1-acyclodextrins, xcex2-cyclodextrins, and xcex3-cyclodextrins. The preferred cyclodextrins are: partially alkyl substituted xcex1-cyclodextrins; partially alkyl substituted xcex2-cyclodextrins; partially alkyl substituted xcex3-cyclodextrins; and combinations thereof The more preferred cyclodextrins are: partially methyl substituted xcex1-cyclodextrins; partially methyl substituted xcex2-cyclodextrins; partially methyl substituted xcex3-cyclodextrins; and combinations thereof. The most preferred cyclodextrins are partially methyl substituted xcex2-cyclodextrins. The cyclic oligosaccharides having a hydrophobic cavity, such as cycloinulohexose, cycloinuloheptose, and cycloinuloctose, are described by Takai et al in Journal of Organic Chemistry, 59(11), 2967-2975 (1994). The calyxarenes useful in the method of the present invention are described in U.S. Pat. No. 4,699,966. The cavitands useful in the method of the present invention are described in Italian patent application No. 22522 A/89 and by Moran, et al., in Journal of the American Chemical Society, 184, 5826-28 (1982).
The amount of optional transport agent to be used is partly determined by the composition of the transport agent. If the transport agent is a cyclodextrin, the weight ratio of cyclodextrin to insoluble substance (e.g., a fluorinated monomer) may range from 1:1000 to 10:100 and is preferably from 1:100 to 5:100, more preferably, from 1:100 to 2:100. The lower limit is determined by such things as the desired rate of transport. The upper limit is determined by the required stability of the aqueous system. If the transport agent is a solvent or monomer, the ratio of transport agent to insoluble substance is less critical, and will depend upon the desired particle morphology. If a solvent is used, the ratio of solvent to pore filler may, for example, range from 1:10 to 10:1. A monomer may be used as the transport agent. The amount of monomer used will be determined by the desired amount of polymeric phase, and by whether additional monomer will be used in forming that polymeric phase.
In the method of the present invention, free radical polymerization may be used to prepare the polymer that will become the major component of the polymeric phase of the porous particles. In that case, the polymer is formed by the polymerization of ethylenically unsaturated monomers, and the polymerization is initiated by decomposition of an initiator to form free radicals. The monomers from which the addition polymer is formed may be monoethylenically unsaturated. The polymer may contain, as polymerized units, one or more monoethylenically unsaturated monomers. Examples of these monoethylenically unsaturated monomers include: C1-C22 linear or branched chain alkyl (meth)acrylates, bornyl (meth)acrylate, and isobornyl (meth)acrylate; hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate; (meth)acrylamide or substituted (meth)acrylamides; styrene or substituted styrenes; butadiene; vinyl acetate or other vinyl ester; vinyl chloride; vinylidene chloride; N-butylaminoethyl (meth)acrylate, N,N-di(methyl)aminoethyl (meth)acrylate; monomers containing xcex1,xcex2-unsaturated carbonyl functional groups such as fumarate, maleate, cinnamate and crotonate; and (meth)acrylonitrile. Used herein, the word fragment xe2x80x9c(meth)acrylxe2x80x9d refers to both xe2x80x9cmethacrylxe2x80x9d and xe2x80x9cacrylxe2x80x9d. For example, (meth)acrylic acid refers to both methacrylic acid and acrylic acid, and methyl (meth)acrylate refers to both methyl methacrylate and methyl acrylate.
Acid-functional ethylenically unsaturated monomer may also be present in the aqueous emulsion polymer. Acid-functional monomers useful in the present invention include, for example, (meth)acrylic acid, itaconic acid, crotonic acid, phosphoethyl (meth)acrylate, sulfoethyl (meth)acrylate, 2-acrylamido-2-methyl-1-propanesulfonic acid, fumaric acid, maleic anhydride, monomethyl maleate, and maleic acid.
Optionally, at least one multi-ethylenically unsaturated monomer may be incorporated into the polymer to provide crosslinking. Useful milti-ethylenically unsaturated monomers include, for example, allyl (meth)acrylate, diallyl phthalate, 1,4-butylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and trimethylolpropane tri(meth)acrylate. The multi-ethylenically unsaturated monomer may be present at 0% to 100%, preferably 0% to 25%, more preferably 0% to 10%, and most preferably 0% to 5% by weight, based on total weight of monomers.
Suitable fluorinated monomers include, but are not limited to: fluoroalkyl (meth)acrylate; fluoroalkylsulfoamidoethyl (meth)acrylate; fluoroalkylamidoethyl (meth)acrylate; fluoroalkyl (meth)acrylamide; fluoroalkylpropyl (meth)acrylate; fluoroalkylethyl poly(alkyleneoxide) (meth)acrylate; fluoroalkylsulfoethyl (meth)acrylate; fluoroalkylethyl vinyl ether; fluoroalkylethyl poly(ethyleneoxide) vinyl ether; pentafluoro styrene; fluoroalkyl styrene; vinylidene fluoride; fluorinated xcex1-olefins; perfluorobutadiene; 1-fluoroalkylperfluorobutadiene; (xcfx89-H-perfluoroalkanediol di(meth)acrylate; and xcex2-substituted fluoroalkyl (meth)acrylate. Preferred fluorinated monomers have a fluoroalkyl group having from 4 to 20 carbon atoms.
In addition silane and siloxane functional monomers such, for example, octamethyl tetracyclosiloxane (known as D4), may be incorporated into the polymer.
The monomers used to prepare the polymer that will form the polymerized phase of the porous particles of the present invention may also be used as xe2x80x9cpreseed monomersxe2x80x9d and xe2x80x9cseed monomersxe2x80x9d to form the pre-seed and seed particles, respectively, of the present invention.
Initiation of free radical polymerization may be carried out by the thermal decomposition of free radical precursors, also called initiators herein, which are capable of generating radicals suitable for initiating addition polymerization. Suitable thermal initiators such as, for example, inorganic hydroperoxides, inorganic peroxides, organic hydroperoxides, and organic peroxides, are useful at levels of from 0.05 percent to 5.0 percent by weight, based on the weight of monomers. Free radical initiators known in the art of aqueous emulsion polymerization include water-soluble free radical initiators, such as hydrogen peroxide, tert-butyl peroxide; alkali metal (sodium, potassium or lithium) or ammonium persulfate; or mixtures thereof. Such initiators may also be combined with reducing agents to form a redox system. Useful reducing agents include sulfites such as alkali metal meta bisulfite, or hyposulfite, sodium thiosulfate, or sodium formaldehyde sulfoxylate. The free radical precursor and reducing agent together, referred to as a redox system herein, may be used at a level of from about 0.01% to 5%, based on the weight of monomers used. Examples of redox systems include t-butyl hydroperoxide/sodium formaldehyde sulfoxylate/Fe(III) and ammonium persulfate/sodium bisulfite/sodium hydrosulfite/Fe(III). Preferred halogenated initiators include trichloroacetyl peroxide, bis(perfluoro-2- propoxy propionyl peroxide, perfluoropropionyl peroxide, perfluoroazoisopropane, and hexafluoropropylene trimer radical. The choice of polymerization temperature depends upon free radical initiator decomposition constant.
Formation of the polymer of the present invention may alternatively be achieved by condensation polymerization. Typically, a condensation polymer is formed as the product of reaction between a first multifunctional monomer and a second multifunctional monomer. An example of such a reactive pair is paraphenylene diisocyanate and hexamethylene diamine. Crosslinking may be achieved by incorporating, for example, trifunctional monomers such as diethylene triamine. Other suitable monomers and methods for preparing condensation polymers therefrom can be found in U.S. Pat. No. 4,360,376 and U.S. Pat. No. 3,577,515. The preferred method for preparing the condensation polymers is to first incorporate the first multifunctional monomer into the droplet medium, and then add the second multifunctional monomer. Reaction to form the condensation polymer then occurs as the second multifunctional monomer reaches the surfaces of the droplets. Because these reactions are usually fast, the polymer is formed at the interface between droplet and water. As such, this preferred form of condensation reaction is known in the art as xe2x80x9cinterfacial polycondensationxe2x80x9d.
Replacement gases include argon, helium, nitrogen, oxygen, carbon dioxide, and mixtures thereof. Air is a specific example of such a mixture. Air includes nitrogen, oxygen, and carbon dioxide at approximately 78, 21, and 0.03 percent by volume, based on the volume of air. During complete or partial removal of the fugitive substance, a replacement gas may be introduced to the pressure vessel to at least partially replace the exiting fugitive substance.
Once the aqueous dispersion of a plurality of porous particles of the present invention has been prepared, the pore filling phase typically remains for at least one hour at one atmosphere, preferably for at least week, more preferably for at least one month, and most preferably for at least one year.
Unwanted components of the aqueous dispersion of the present invention may removed by techniques well known in the art, such as, for example, diafiltration.
When a single polymeric phase is present in the porous particles of the aqueous dispersion of the present invention, that polymeric phase has the following xe2x80x9ceffective Tgxe2x80x9d range: the lower limit is typically at least 5xc2x0 C. above the freezing point of the aqueous phase, and, preferably, the lower limit of the effective Tg range is 10xc2x0 C., more preferably 30xc2x0 C., and most preferably 50xc2x0 C.; the upper limit is only limited by the Tg of the polymeric phase, and is typically 350xc2x0 C., preferably 250xc2x0 C., more preferably 175xc2x0 C., and most preferably 125xc2x0 C. When there is more than one polymeric phase in the porous particles of the present invention, at least one polymeric phase must have an effective Tg within the ranges just stated.
The same ranges for effective Tg stated supra apply to the polymeric phase of the porous particles of the present invention after isolation from the aqueous dispersion. When the porous particles are isolated from the aqueous phase, it is preferred that such isolation occur at a temperature at least 3xc2x0 C. below the effective Tg of the polymeric phase having the highest effective Tg. If, however, isolation from the aqueous phase is carried out after combining the aqueous dispersion of the present invention with a second aqueous dispersion containing, for example, a polymer having an effective Tg greater than that of the polymeric phase of the porous particles of the present invention, it may be possible to carry out isolation at a temperature greater than the effective Tg of the polymeric phase of the porous particles and, preferably, less that the effective Tg of the polymer of the second aqueous dispersion. Alternatively, the polymer having as effective Tg greater than that of the polymeric phase of the porous particles may be added to the aqueous dispersion in any form that will disperse, e.g., as a solid dispersible powder.
The plural porous particles may be isolated from the aqueous dispersion of the present invention by methods well known in the art, including, for example, filtration followed by air, or oven drying; formation of a film of the aqueous dispersion, followed by air, or oven drying; coagulation followed by air, or oven drying; and spray drying. Isolation may include the further step of combining the aqueous dispersion of the present invention with at least one second aqueous dispersion or solution containing at least one component, for example, a polymer, or other additive. Alternatively, such components may be added as the pure component or as mixtures of more than one component. The isolation may further include a step of at least partially removing residual ingredients and impurities (e.g., initiator fragments and dispersing agents) from the aqueous phase by techniques well known in the art, such as diafiltration. When the porous particles of the present invention are isolated as a powder (i.e., the neat particles), isolation should be carried out at a temperature below the Tg of at least one polymeric phase. The isolated porous particles have the same, or slightly smaller, average particle size and the same PSD polydispersity as they had in their aqueous dispersions. The average particle size of the isolated porous particles might, for example, be slightly smaller than the average particle size for the porous particles before isolation from the aqueous dispersion if the polymeric phase of the porous particles had been swollen with a substance (e.g., fugitive substance, or water) that later evaporated, or was otherwise removed, from the particles during or after isolation.
The plural porous particles of the present invention are useful as components of many systems. A non-exhaustive list of such systems includes, for example, paints, coatings, inks, sunscreens, and paper. The plural porous particles of the present invention are further useful as image enhancers for ultrasonic imaging. Though not wishing to be bound be any particular theory, it is believed that the differences in elasticity among the replacement gas (e.g., air; fluorocarbon), the polymeric shell of the porous particle, and water result in ultrasonic scattering which enhances contrast during, for example, medical imaging.